Recently, it has been pointed out that ultraviolet ray in the UV-A region (320 to 400 nm) penetrates deep into the skin, becoming the main causative factor for photoaging and skin cancer. In view of this, the demand has also risen for cosmetics with a UV-protective effect in the UV-A region.
Conventionally, inorganic powders such as titanium dioxide and zinc oxide have been used for protection against ultraviolet ray. Among them, zinc oxide has recently been frequently used in cosmetics for its relatively high shielding ability against not only the UV-B region (290 nm to 320 nm) but also the UV-A region.
In order to increase the UV-protective effect, fine zinc oxide particles having an average particle diameter of 0.1 μm or less are used. However, these fine zinc oxide particles are prone to aggregate, resulting in poor dispersibility, and when they are blended in a large amount, the resulting cosmetic has poor spreadability, creating unnaturally white spots. For these reasons, the above fine zinc oxide particles are impractical also in terms of usability.
In order to solve the aforementioned problem, for example, there is proposed a stable sunscreen cosmetic composition having an excellent powder dispersibility, a high UV-protective effect, favorable transparency, and a favorable feeling upon application obtained by using a powder produced by subjecting fine zinc oxide particles of 0.1 μm or less to surface treatment with silicic anhydride and then to silicone treatment, and specific polyoxyalkylene-modified polysiloxane (refer to Patent Document 1). However, the above sunscreen cosmetic composition does not have sufficient smooth spreadability, resulting in a poor feeling upon application, and it has insufficient resistance to sweat and water from the external environment. Thus, it is not satisfactory from the viewpoint of cosmetic durability.
Also, a skin external agent in which transparency and a UV-protective effect are improved by using a flaky powder of zinc oxide is proposed (refer to Patent Document 2). However, such the skin external agent has poor dispersibility and is unsatisfactory in terms of unnaturally white spots resulting from blending of the powder and also the feeling upon application.
As a solution to the aforementioned problem, for example, there is proposed a cosmetic composition having an excellent feeling to the touch, high transparency, and an excellent protective capacity against ultraviolet ray obtained by using a flaky powder of zinc oxide having an average particle diameter of 0.1 to 1 μm and fine metal oxide particles having an average particle diameter of 0.001 to 0.1 μm at a specific ratio (refer to Patent Document 3).
Also, there is proposed a cosmetic composition having high transparency and a high UV-protective effect which spreads out well on the skin obtained by using flaky powder of zinc oxide having an average particle diameter of 0.1 to 1 μm and spindle-like or needle-like fine titanium dioxide particles having a minor axis of 0.05 to 0.1 μm and a major axis of 0.01 to 0.5 μm (refer to Patent Document 4).
Further, there is proposed a water-in-oil emulsified cosmetic composition having an excellent feeling upon application and excellent durability obtained by using flaky powder of zinc oxide having an average particle diameter of 0.1 to 1 μm, an average particle thickness of 0.01 to 0.2 μm, and an average aspect ratio of 3 or more, an ether-modified silicone, and a silicone oil (refer to Patent Document 5).